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Student Research Finds Better Way to Build Green Roofs

“Think globally, act locally” is more than just an environmental catchphrase – it’s also a sound way to approach green roof construction, according to James McDonald, who earned his B.S. in biology at Loyola Marymount University in May.

McDonald’s research into green roof technology during his undergraduate years strongly suggests that indigenous plants fare better in Southern California’s climate, enhancing the benefits of having a green roof atop a building.

McDonald presented his findings at LMU’s fifth annual Undergraduate Research Symposium. His work focused on comparing the most common type of plant used on green roofs – sedums, shown in the top half of the picture – with an indigenous type of plant, dudleyas. “The basis of the project was to test the thermo-tolerance of the two genera of plants,” McDonald said.

His research indicated that the local dudleyas, a type of succulent plant, were better suited to higher temperatures and conditions that would be typical in Southern California.

Sedums are the most-used plants in green roofs because they grow relatively well in areas where green roof industries first took off, in climates that are less arid than Southern California, said Philippa Drennan, professor of biology at LMU and adviser on McDonald’s project. “Europe’s got a big green roof culture and sedums grow quite well in that area. Also north of [Southern California], getting up toward Canada, or even toward the northeast of the United States, sedums are very good because they tend to grow in cooler areas,” she said.

McDonald was a junior when he joined Drennan’s ongoing research into green roofs, which provide a natural alternative to other insulation materials. They also help reduce stormwater runoff and provide habitat for birds and insects.

In high-heat areas such as Southern California, temperatures on top of most buildings will get so high that plants are prone to dry out and die. “We found that the dudleyas were able to withstand higher temperatures,” McDonald said, “and at the highest temperatures more cells were living in the dudleyas than in the sedums.”

But survivability wasn’t McDonald’s only research concern. “When you put things that are non-native into areas, it sometimes has detrimental effects. It doesn’t support the insects or birds or bees of that environment,” he said, adding that the local dudleyas would have a better chance to develop as an ecosystem on rooftops and be an effective green roof.